In the growing computerization society, there is a serious problem of the leakage of confidential information, and the development of the technology for preventing information leakage is strongly demanded. For example, relating to digital data, the technology of encrypting data has been developed not reveal the contents of the data to the third parties although the data is received by the third parties, and has been utilized as effective means for preventing information leakage.
On the other hand, the effective technology of preventing information leakage of printed matter on paper media etc. has not been developed or commercialized. Practically, it is said that about half of the information leakage relates to printed matter, and the development of the technology for preventing the information leakage from printed matter as well as digital data is also demanded seriously. Practical examples of the demand for the measures against the information leakage from printed matter can relate to a bill issued when goods are purchased, a specification of a credit card etc., a medical record in hospital, a test record in school, books, etc.
In encrypting the data on printed matter, a marker recognition technology for identifying an encrypted part as well as an encrypting method is required.
As the first conventional technique relating to the encrypting technology and the marker recognition technology for printed matter, a two-dimensional code technique for representing binary data as an image and printing it on an analog medium is well known. Representing data as an image in this technique is realized by expressing binary data as white and black squares in a specified size and arranging the squares in matrix form. Furthermore, to indicate the position of the image in a decoding operation, a positioning marker is attached to printed matter at the position specified in the matrix. On the basis of the positioning marker, the image is captured by a scanner, a camera, etc., and embedded information can be decoded.
As the second conventional technique relating to the marker recognition technology, a marker indicating the position of an encrypted area is arranged at the periphery of the area. As with the case of the two-dimensional code, the coordinates of an encrypted area can be obtained by a reader detecting the marker arranged at the encrypted area, thereby performing the decoding process.
When an encrypted image is decoded, it is necessary to recover the positioning marker attached at the periphery of the encrypted area to the data before the marker is attached. As the third conventional technique relating to the process, there is the technology of recovering the marker at the periphery to the state before attaching the marker by the interpolating process using the value of the pixel adjacent to the information and the marker embedded in the header of an image file.
Relating to each of the above-mentioned techniques, the following patent documents are disclosed.    Patent Document 1: Japanese Patent No. 2938338    Patent Document 2: Japanese Laid-open Patent Application No. 2008-301044    Patent Document 3: Japanese Laid-open Patent Application No. 2009-232233
In each of the above-mentioned conventional techniques, there occurs a problem when an area to be encrypted is small when, for example, only one line of sentence described by small characters, only one cell in a table is to be encrypted, etc.
Since the amount of information to be embedded for each unit area is limited in the above-mentioned first technique, the technique cannot be applied.
There has been the problem that the markers cannot be detected because the markers arranged at the peripheries of areas overlap each other in the above-mentioned second and third technique.